The emerging concerns over the depletion of the fossil fuel sources and the impact of greenhouse gas emission have created great demands for the development of large-scale energy storage systems for electric vehicles (EV) and renewable energy resources such as wind and solar. Lithium-ion batteries are considered as one of the most attractive technologies for rechargeable energy storage for electrical vehicles due to their high energy density and long service life. However, there is still great concern about the cost of Li-ion batteries and the potential limit of Li supplies available in terrestrial reserves, especially for the large-scale energy storage applications for renewable energy and grid. Therefore, alternative energy storage mechanisms and devices using abundant and environmentally friendly materials are highly desirable.
Sodium-ion batteries have been discussed in the literature for some time. A battery that uses sodium ions instead of lithium ions is attractive because it could be potentially much cheaper and safer, and it is more environmentally benign. A sodium ion storage mechanism is also scientifically interesting and challenging because sodium ions are about 70% larger in radius than that of lithium ions. This makes it difficult to find a suitable host material to accommodate the sodium ions and allow reversible and rapid ion insertion/extraction.
In the literature, hard carbon based negative electrodes have been reported to deliver a capacity of 300 mAh g−1 through Na ion insertion/deinsertion reaction. However, few studies have been reported for the Na-ion battery cathode materials with decent performance. For example, fluorophosphates materials were developed as a cathode material for Na-ion battery. Barke et al. studied a hard carbon/NaVPO4F battery demonstrating a specific capacity of 79 mAh g−1 from NaVPO4F based in the initial cycle, but less than 50% of the original capacity after 30 cycles. Most of the research on cathode materials for rechargeable sodium batteries has been focused on the manganese oxides because of their large-size tunnels for Na ion insertion and deinsertion. Morales et al. reported that layered P2-Na0.6MnO2 can deliver 150 mAh g−1 first cycle capacity, but this material exhibited a poor capacity retention capability with more than 50% of capacity loss after only ten cycles. For most layered and tunnel-type manganese oxides, the main cause of the structural instability during repetitive cycling is the inability to accommodate the Jahn-Teller distortion following the reduction of Mn(IV) to Mn(III) within the rigid close packed oxide ion structures. A similar fading mechanism has been identified for the capacity degradation of the layered LiMnO2 and spinel LiMn2O4. A recent study by Kim and Johnson reported that a layered Na—Ni—Mn oxide with Li doping exhibited a capacity of about 95 mAhg-1 with good capacity retention over 50 charge-discharge cycles. However, this kind of material still needs 20% molar ratio Li to stabilize the structure during cycling. A Li-free cathode material, with a much higher capacity and much longer cycle life is desired.
Recently, MnO6 octahedra and MnO5 square pyramids are found to form large, double ion channels in orthorhombic Na4Mn9O18 (Na0.44MnO2) and offer better cycling stability due to the ability to tolerate some stress during structural changes, as shown in FIG. 1A. There have been numerous studies on the structures and electrochemical properties of such materials, but the insertion/deinsertion behavior of sodium ions has not been well studied. Sauvage et al. have shown reversible insertion and deinsertion of sodium ion in pure Na4Mn9O18 prepared with an initial capacity of about 80 mAh/g at a C/10 rate, but the cycling capacity faded rapidly with cycling and only half of the initial capacity was retained after 50 cycles. Compared with lithium-ion, the inferior sodium inserting/deinserting performance is mainly due to the failure of these Na4Mn9O18 to accommodate the structural change during the insertion/extraction of large sodium ions, leading to slow kinetics and structure degradation.